Integrated circuit devices, such as processors, microcontrollers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), programmable logic arrays (PLAs), complex programmable logic devices (CPLDs), and field programmable gate arrays (FPGAs), can include numerous types of discrete circuit components, including transistors, resistors, and capacitors, as well as other components or circuit structures. Device designers and manufacturers routinely attempt to increase the speed and performance of such integrated circuit devices while at the same time reducing die and/or package size and maintaining device reliability. However, the presence of hundreds of thousands or millions of closely-spaced transistors and other discrete components exhibiting sub-micron dimensions and operating at high clock rates inevitably causes the device to exhibit high power dissipation and heating.
High temperatures can damage or destroy integrated circuit components, and operation of an integrated circuit at a temperature above a certain level can be indicative of design or manufacturing defects in the device. Consequently, many systems, devices, and techniques exist for measuring and monitoring integrated circuit temperature.
When temperature is measured in an integrated circuit, a semiconductor junction is often used in the process. By manipulating the currents and the current densities through the junction, changes in voltage can be measured across the junction. A change in voltage at two current densities across the junction can be measured and used by a temperature sensor to calculate temperature. Most junctions employed for this purpose are parasitic vertical PNP silicon based transistors. However, it should be appreciated that NPN transistors or even diodes may be used instead.
The classic transistor equation determines a change in the base emitter voltage (ΔVBE) for a PNP transistor as follows:
          ⁢          V      BE        =      η    ⁢          kT      q        ⁢          ln      [                        I                      C            ⁢                                                  ⁢            2                                    I                      C            ⁢                                                  ⁢            1                              ]      where η is a non-ideality constant substantially equivalent to 1.00 or slightly more/less, k is the well known Boltzmann's constant, q is the electron charge, T is the temperature in Kelvin, IC1 and IC2 are collector currents that are present at the measurement of a first base-emitter voltage and a second base-emitter voltage respectively.
There are two basic types of temperature sensors that utilize the concept of the diode equation: “diode mode” sensors and “transistor mode” sensors. Diode mode sensors operate on the assumption that a ratio of collector currents tends to be relatively equivalent to a ratio of known emitter currents (IE). Hence, for a diode mode sensor, the diode approximation of the transistor equation (or “diode equation” for short) is approximated by:
                  ⁢              V        BE              =          η      ⁢              kT        q            ⁢              ln        [                              I                          E              ⁢                                                          ⁢              2                                            I                          E              ⁢                                                          ⁢              1                                      ]              ;            where      ⁢                          ⁢                        I                      C            ⁢                                                  ⁢            1                                    I                      C            ⁢                                                  ⁢            2                                =                  I                  E          ⁢                                          ⁢          1                            I                  E          ⁢                                          ⁢          2                    
In both diode mode and transistor mode sensors, a problem arises in measuring the voltage across the junction, because the actual voltage across the junction is never measured due to the fact that error terms are introduced by series resistances in the measurement path to and from the junction.
An exemplary circuit diagram of FIG. 1 illustrates one temperature measuring circuit that experiences this problem of measurement-induced error. In FIG. 1, a temperature sensor supplies a current to the emitter of a PNP transistor, and then receives an input current from the base of the same transistor. A base-emitter voltage is generated across the base-emitter junction of the transistor. However, due to the series resistance of the measurement lines, the temperature sensor actually measures a slightly different voltage than what is present across the base-emitter junction of the transistor. The series resistance is represented by resistor RE in series between the temperature sensor and the emitter of the PNP transistor and resistor RB in series between the base of the PNP transistor and the temperature sensor. The presence of these series resistances introduces error.
In the past, especially in integrated circuit production techniques at the 0.09 micron level and larger, this type of measurement-induced error could be ignored by a temperature sensor because accuracy needs were not as stringent. However, at smaller circuit production techniques, this error becomes larger and must be dealt with. A typical way to deal with this was generally to add an offset—either a resistance offset, a temperature offset, or a software offset that helps compensate for the error that is induced by the measurement. In one case, the amount of offset would be determined by simply multiplying a typical resistance of the circuit by a typical current through the circuit. In another case, the amount of offset would be determined by multiplying a typical resistance by the actual current. In either case, the offset only works in conditions where the error term has no significant temperature dependency. In the past, there was only a very small temperature dependency in the measurement errors. However, at smaller integrated circuit sizes, starting at around 65 nanometers, there is large temperature dependence in the resistances induced by measurement. As a consequence, simply dealing with these resistances through the use of some sort of offset does not yield an accurate temperature measurement at a variety of temperatures, and therefore the overall system accuracy of a temperature system suffers. A further problem exists in that some portions of the error term are non-obvious, and thus hard to identify.
As mentioned in U.S. Pat. No. 7,333,038, there are well-known techniques for dynamically canceling the effects of this series resistance on a real time basis. These techniques are only suitable for cases when the sensing junction is an actual diode or a transistor that substantially behaves like a diode, i.e. has high and constant current gain. For the small geometry processes it has been shown that the temperature sensing transistors do not behave like simple diodes, hence making these dynamic resistance correction techniques largely useless.
Offsets may have different causes in a signal processing chain of an integrated circuit. Fluctuations in the supply voltage and temperature drift effects, fluctuations in process parameters during fabrication, and matching problems between electronic components in the case of differential signal routing contribute, for example, to the occurrence of offsets.
Furthermore, in order to measure an integrated circuit temperature with the above equation, a different current is passed through the same diode, or likewise a same current is passed through two different diodes, whenever the integrated circuit temperature is to be measured. While there are many ways to measure the two voltages of two diodes at different current densities and performing subtraction and amplification in the prior art by using an analog amplifier, such methods result in error introduced due to device mismatch.
Previous temperature sensors using analog components are subject to errors arising from device mismatch. For example, if a current density ratio of 16 is used to generate the two diodes voltages, the resulting difference voltage will be only 26 mV*ln(16) or about 72 mV. This is a difficult quantity to process with an analog amplifier where typical offsets may be on the order of 10 to 20 mV. The offset of the amplifier will then present an error to the system that will vary from part to part and degrade the accuracy of the measurement irrecoverably. For this reason, the offset of the amplifier is minimized at the potential cost of complexity, die area, and power.
A discrete-time switched-capacitor amplifier can overcome offset issues by sampling and canceling the offset in between sampling and amplifying the diode voltages. This method has proven to be suitable for highly accurate sensors but at the cost of considerable complexity.
Therefore, a need exists for a temperature measuring system and a temperature measuring method that eliminates error from device mismatch, and measures temperature regardless of the offset of the amplifier with a lower cost.